Anion Binding Modes in meso-Substituted Hexapyrrolic Calix[4]pyrrole Isomers

We
report on the synthesis of a new receptor for anions, meso-substituted hexapyrrolic calix[4]­pyrrole 1. The calix[4]­pyrrole’s
core features two additional pyrrole
side-arms suspended above or below the calix[4]­pyrrole core. This
hexapyrrolic calix[4]­pyrrole 1 is formed as cis- and trans-configurational isomers, the structures
of which have been determined by single crystal X-ray diffraction.
The anion binding experiments revealed interesting difference in the
binding mode: The cis-1 isomer binds
anions in a mixed binding mode featuring a combination of hydrogen
bonding and anion−π interactions resulting in an unexpected
strong binding. On the other hand, the trans-1 isomer displays only hydrogen bonding and lower affinity
for anions. This is unexpected as one would assume both isomers to
display the same binding modes. Overall, the titrations of 1 using UV spectrophotometry and NMR titrations by anions reveal that cis-isomer 1 displays higher affinity (105–106 M–1) and cross-reactivity
for anions, while the trans-isomer 1 shows a more selective response to anions. Such differences in binding
mode in configurational isomers are so far unexplored and a feature
deserving further study.

本研究报告了一种新型阴离子受体的合成，即中位取代的六吡咯基卡利克[4]吡咯1。该卡利克[4]吡咯的核心结构包含两个额外的吡咯侧链，分别悬挂于卡利克[4]吡咯核心之上或之下。该六吡咯基卡利克[4]吡咯1以顺式和反式构型异构体的形式形成，其结构通过单晶X射线衍射技术得以确定。阴离子结合实验揭示了有趣的结合模式差异：顺式-1异构体以氢键和阴离子-π相互作用的混合结合模式结合阴离子，从而产生了出乎意料的强结合力。另一方面，反式-1异构体仅表现出氢键和较低的结合亲和力。这种差异令人惊讶，因为人们可能会预期两种异构体表现出相同的结合模式。总体而言，通过紫外分光光度法和阴离子滴定的核磁共振滴定，对1的滴定实验表明，顺式异构体1对阴离子的亲和力（105–106 M–1）和交叉反应性较高，而反式异构体1则对阴离子的选择性响应更为显著。此类构型异构体在结合模式上的差异迄今为止尚未被充分探索，因此值得进一步研究。